I. Definitions
As used herein, the phrase “group III-V” refers to a compound semiconductor that includes a group V element and at least one group III element. Moreover, the phrase “III-Nitride or III-N” refers to a compound semiconductor that includes nitrogen (N) and at least one group III element including aluminum (Al), gallium (Ga), indium (In), and boron (B), and including but not limited to any of its alloys, such as aluminum gallium nitride (AlxGa(1-x)N), indium gallium nitride (InyGa(1-y)N), aluminum indium gallium nitride (AlxInyGa(1-x-y)N), gallium arsenide phosphide nitride (GaAsaPbN(1-a-b)), and aluminum indium gallium arsenide phosphide nitride (AlxInyGa(1-x-y)AsaPbN(1-a-b)), for example. III-Nitride also refers generally to any polarity including but not limited to Ga-polar, N-polar, semi-polar or non-polar crystal orientations. A III-Nitride material may also include either the Wurtzitic, Zincblende or mixed polytypes, and may include single-crystal, monocrystalline, polycrystalline, or amorphous structures.
Also as used herein, the phrase “group IV” refers to a semiconductor that includes at least one group four element including silicon (Si), germanium (Ge), and carbon (C), and also includes compound semiconductors such as SiGe and SiC, for example. Group IV may also refer to a semiconductor material which consists of layers of group IV elements or doping of group IV elements to produce strained silicon or other strained group IV material. In addition, group IV based composite substrates may include silicon on insulator (SOI), separation by implantation of oxygen (SIMOX) process substrates, and silicon on sapphire (SOS), for example. Moreover, a group IV device may include devices formed using standard CMOS processing, but may also include NMOS and PMOS device processing.
Furthermore, as used herein, the terms “LV device,” “low voltage semiconductor device,” “low voltage transistor, low voltage switch” and the like, refer to a low voltage device, with a typical breakdown voltage rating less than an “intermediate device,” as described below. The LV device can include any suitable semiconductor material that forms a field-effect transistor (FET) such as an insulated-gate FET (IGFET), for example. Suitable semiconductor materials include group IV semiconductor materials such as Si, strained silicon, SiGe, SiC, and group III-V materials including III-As, III-P, III-Nitride or any of their alloys.
Additionally, the term “intermediate device, intermediate transistor and intermediate switch” refers to a device with a typical breakdown voltage greater than the LV device and less than a “primary device”. The “primary device, primary transistor or primary switch” refers to a device with a typical breakdown voltage greater than both the intermediate device and the LV device.
II. Background Art
In high power and high performance switching applications, group III-V field-effect transistors (FETs) and high mobility electron transistors (HEMTs), such as III-Nitride FETs and III-Nitride HEMTs, are often desirable for their high efficiency and high-voltage operation. Moreover, it is often desirable to combine such group III-V transistors with other FETs, such as group IV FETs, to create high performance composite switches.
In power management applications where normally OFF characteristics of power devices are desirable, a depletion mode (normally ON) III-Nitride or other group III-V transistor can be cascoded with an enhancement mode (normally OFF) low-voltage (LV) group IV transistor, for example a silicon FET, to produce an enhancement mode (normally OFF) composite power switch. However, the performance of the composite switch can be limited by the on-state and voltage breakdown characteristics of the LV group IV transistor used. In particular, the breakdown voltage for a given on-state resistance of the LV group IV transistor may be insufficient to support the required pinch-off voltage required to maintain the group III-V transistor in a satisfactorily OFF condition.